"Passive" restraint apparatus, which require no action by a vehicle occupant to make operative, as contrasted to "active" restraint apparatus requiring a conscious effort by an occupant to become operative (such as conventional seat belts), are now commonly being included as standard equipment in motor vehicles due to government agency, insurance industry, and consumer pressures. Experience and testing have shown that the most effective passive occupant restraint approach is to abruptly (in a matter of milliseconds) inflate a safety bag automatically in response to a sudden change in vehicle velocity occasioned by a high impact accident. The inflated safety bag pins the occupant in his/her seat, well cushioned from the effects of the accident for an interval sufficient to prevent serious injury.
The mechanics for timely inflating a safety bag are basically of two types. One type involves storing a liquefied gas under high pressure in a reservoir. On impact, the liquefied gas is released, which then converts to a gas, inflating the safety bag. The approach has numerous disadvantages. The bulk and weight of the components for handling the gas-producing liquid makes it difficult and expensive to package the safety apparatus in the steering column and dashboard of motor vehicles. Storage integrity over a long time period of widely varying ambient conditions is tenuous at best. Moreover, the most effective and practical gas-producing liquids are chlorofluorocarbons, such as FREON, usages of which are currently being seriously curtailed due to environmental concerns.
As a consequence, safety bag inflation through the release of liquefied gas has largely been supplanted by the use of ignitable propellants capable of rapidly generating large volumes of gas by exothermic reaction. Heretofore, the propellants have invariably been in a solid or granular form comprised of low energy compositions, such as an alkali metal azide, cupric oxide, boron nitrate, etc. The solid propellants need not be stored under pressure, are stable over time under varying ambient conditions, and are convenient to package in a passive restraint apparatus. There is however a significant drawback to solid propellants of this type, in that the generated gas contains toxic by-products and particulate matter. As a consequence, the safety bag inflating gases must be filtered to remove the toxic by-products and also to catch combusting particulate matter which is capable of burning holes in the safety bag. These filters increase the size, weight, and cost of a passive restraint apparatus.
As an alternative to the use of propellants in solid or granular form as the inflation gas generator, use of liquid propellants for this purpose is being investigated. A viable approach to using a liquid propellant in a safety bag inflater is disclosed in U.S. Pat. No. 5,060,973, issued to Giovanetti. The numerous advantages afforded by the use of liquid propellants in a safety bag inflation apparatus are noted in the Giovanetti patent, the disclosure of which is incorporated herein by reference.